1. Field of the Invention
The invention concerns an x-ray computed tomography (CT) system of the type having a source/detector system to generate tomographic phase contrast or dark field exposures, the source/detector system being arranged on a gantry that can rotate around a system axis.
2. Description of the Prior Art
An x-ray CT system for generating tomographic phase contrast or dark field exposures is known that has a source/detector system mounted on a gantry for rotation around a system axis. The source/detector system of this known x-ray CT system has an x-ray source and a detector arrangement. The x-ray source has a grating structure that emits bands of emission maxima and emission minima of a generated x-ray radiation with a grating period, the bands are arranged like a grating, so a beam fan of x-rays expanding in two planes arises with a maximum fan angle in the rotation plane of the gantry and a Z-angle perpendicular to the rotation plane of the gantry,
The detector arrangement has a number of grating/detector modules arranged parallel to one another, each module having arranged one after the other in the beam direction):
at least one phase grating that generates an interference pattern, and
an analysis grating with a directly connected detector with a number of detector elements to determine phase, average radiation intensity and amplitude of the average intensity of the radiation per detector element given relative displacement of one of the upstream grating structures,
with the grating lines of all grating structures aligned parallel to one another and parallel to the system axis.
Such an x-ray CT system with modular design of the source/detector system composed of a number of grating detector modules that are operated as a Talbot interferometer is known from the disclosure document DE 10 2006 015 358 A1. The geometric conditions that are necessary for the arrangement of the grating structures in order to be able to achieve reasonable measurement results are also described therein. Such geometric conditions include:
                    p                  0          ⁢                                                    =                        p                      2            ⁢                                                                ×                                  ⁢                  1          d                      ,                  ⁢                  p        1            =              2        ×                                            p              0                        ×                          p              2                                                          p              0                        +                          p              2                                                      d      =                                                  1              ×                              d                ≡                                                    1              ×                              d                ≡                                              ⁢          with          ⁢                                          ⁢                      d            ≡                          =                              1            2                    ×                      (                                          p                1                2                                            4                ⁢                λ                                      )                                ,                  ⁢                  h        1            =              λ                  2          ⁢                      (                          n              -              1                        )                              wherein    p0=hereby means the grating period of the source grating G0,    p1=hereby means the grating period of the phase grating G1,    P2=hereby means the grating period of the analysis grating G2,    d=distance of the phase grating G1 from the analysis grating G2 in the fan beam geometry,    d≡=distance of the phase grating G1 from the analysis grating G2 under parallel geometry,    I=distance of the source grating G0 from the phase grating G1,    λ=selected wavelength of the radiation,    h1=web height of the phase grating G1 in radiation direction,    n=index of refraction of the grating material of the phase grating.
Furthermore, it is stated that the line orientation of the gratings G0 through G2 should be fashioned so that the grating lines of all three gratings run parallel to one another. However, this should advantageously not be necessary if these grating lines are oriented parallel or perpendicular to the system axis S.
It has been shown, however, that grating/detector modules arranged at large fan angles show unsatisfactory results given an execution of such a grating-populated source/detector system in which the grating lines are aligned parallel to the system axis and a relatively wide detector is simultaneously used (thus a large fan angle perpendicular to the system axis occurs for the radiation beam that is used).